Author
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Idso, Sherwood |
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Kimball, Bruce |
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Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/23/1996 Publication Date: N/A Citation: N/A Interpretive Summary: As the carbon dioxide content of earth's atmosphere continues to rise in response to the burning of the coal, gas and oil that power the engines of industry, we need to know how this CO2 enrichment of the air will affect the growth of plants. Most investigations of this problem have centered on crops that are planted in the spring and harvested in the fall. Consequently, we have accepted the more difficult challenge of determining the growth response of a long-lived tree whose rootstock provides the basis for the bulk of the citrus industry. Specifically, we have assessed the growth characteristics of eight sour orange trees over the past seven years in what is the longest continuous CO2 enrichment experiment ever to be conducted anywhere in the world. Our data indicate that a 75% increase in the air's CO2 content more than doubles the productivity of the trees, demonstrating that the stimulus provided by extra CO2 can be sustained over rlong periods of time. Our results should encourage other scientists to undertake similar high-risk, long-term studies, as well as enable citrus growers to better plan for the future. Technical Abstract: In July 1987, we planted eight 30-cm-tall sour orange tree seedlings in a field of Avondale loam at Phoenix, Arizona and enclosed them in pairs in clear-plastic-wall open-top chambers. Since 18 November of that year, we continuously pumped ambient air of approximately 400 ppmv CO2 through two of these chambers, while through the other two we continuously pumped air of approximately 700 ppmv CO2. Throughout the ensuing years, we measured the trunk circumference of each tree at the mid-point of each month; and from the end of the third year of the study, we counted the numbers of oranges removed from all trees at the conclusions of each year's harvest. In connection with the last of these harvests, we also measured the diameter of every orange removed from each tree, along with the rind thicknesses of 240 randomly selected oranges from each of the two CO2 treatments. From these data we developed a seven-year history of the growth-promoting effects of atmospheric CO2 enrichment. This history indicates that by the end of the second year of the study, the trunk plus branch volume of the CO2-enriched trees was approximately 2.75 times greater than that of the ambient-treatment trees. Since that time, this factor has dropped to just under 2.0; but the decline in the CO2- enriched/ambient-treatment ratio of trunk plus branch volume has been nearly perfectly offset by the relative fruit production advantage enjoyed by the CO2-enriched trees, which began to manifest itself in year 3 of the study. From year 2 through year 5, for example, the total trunk, plus branch, plus cumulative fruit rind volume of the trees exposed to the extra CO2 was consistently about 2.75 times greater than the corresponding volume of the trees exposed to ambient air. |
